The invention is concerned with the removal of inorganic matter from solid surfaces using supercritical fluids. Methods of the invention can be used for the decontamination of radioactive waste.
The problem of removal of adsorbed metal ions, in particular of radionuclides, from the surface of stainless steel is usually solved by treating the surface with aqueous solutions, foams or suspensions of acids and/or complexones. Such methods are disclosed in (N I Ampelogova, Yu M Simanovskii, A A Trapeznilov: Decontamination in the nuclear power industry. Moscow, Energiozdat, 1982, p 140-152: and Dippel T, Hentschel D, Kunze S: Kerntechnik, 1976, Vol 18, No 12, p 526-531:
The application of these methods makes it possible to remove metal ions from the surfaces but leads to the production of secondary waste, namely acidic solutions containing radionuclides. This can cause a substantial increase of the total volume of wastes. As a result of the use of such a method the radionuclides are transferred to a solution. In order to secure their safe storage it is necessary to convert them into an ecologically benign form, and this causes further problems. Consequently, a method that reduces the volume of secondary waste and facilitates the transformation of the radionuclides into an ecologically safe form will lead to a cheaper and safer decontamination process.
Methods are known for the supercritical extraction of metal complexes with the help of carbon dioxide gas in the presence of complexones (diethyl-dithio carbamates, bis-(trifluoromethyl)-dithio carbamates/K E Laintz, C M Wai, C R Yonker, R D Smith, Extraction of Metal Ions from Solid and Liquid materials by Supercritical Carbon Dioxide, Anal Chem, 1992, Vol 64, p 2875-78; tributyl phosphate/Y Lin, R D Brauer, K E Laintz, C M Wai, Supercritical Fluid Extraction of Lanthanides and Actinides from solid Materials with fluorinated xcex2-diketones, Anal Chem 1993, Vol 65, p 2549-2551; triazolo-crown ethers/S Wang, S Elshani, C M Wai, Selective Extraction of Mercury with Ionisable Crown Ethers in Supercritical Carbon dioxide, Anal Chem 1997/), and also xcex2-diketones/Y Lin, C M Wai, F M Jean, R D Brauer, Supercritical Fluid Extraction of Thorium and Uranium Ions from solid and Liquid Materials with Fluorinated xcex2-diketones and Tributyl Phosphate, Environ Sci Technol, 1994, Vol 28, No 6, p 1190-93.; Wai C M, Smart N G, Phelps C, Extraction of Metals Directly from Oxides, U.S. Pat. No. 5,606,724 A Publ Feb. 25, 1997.
In using a xcex2-diketone and tributyl phosphate a sample of the material (sand, paper etc) may be covered with an acetate buffer solution with pH=4.0 which contains a metal (10 xcexcg). xcex2-diketone (40-80 xcexcmol) and tributyl phosphate (40 xcexcmol) are added. The sample is placed in supercritical carbon dioxide which contains methanol or water where it is kept for 10 minutes at 60xc2x0 C. and 150 bar. The flask is then washed with 10 flask volumes of clean carbon dioxide and the extract is collected in water.
This method has the following disadvantages: For complete extraction of the metal a large excess of xcex2-diketone is required (400-1000 mol per 1 mol of metal) and the extraction must be done from a buffer solution. It is impossible to achieve a sufficiently complete extraction of the salt formed by the metal from a stainless steel surface. Extraction of transuranium elements has not been reported so far.
The aim of the method of the invention is to remove from the surface of solid bodies metallic contaminants, including radioactive ones. It is important that the contaminating metals will be removed from the surface, irrespective of their chemical form (ie salts, oxides, etc).
A method is proposed for the removal of inorganic matter from a solid surface, which may be contaminated with one or more radionuclides, the method comprising contracting the solid surface with a supercritical fluid, for instance supercritical carbon dioxide, which contains an acidic ligand and an organic amine, and collecting the resultant extract in a suitable solvent
The method of the present invention does not require preliminary covering of the surface with a buffer solution. It is possible to extract radionuclides in practically all their chemical forms (chlorides, nitrates, sulphates, oxides, etc).
An acidic ligand of use in the present invention is one which can be deprotonated. An example of an acidic ligand is a xcex2-dicarbonyl compound, for instance a xcex2-diketone or a xcex2-keto-ether.
A preferred xcex2-dicarbonyl compound has the formula 
where R1, R2, Rxe2x80x22 and R3 are each independently selected from hydrogen, alkyl, aryl, fluorine-substituted alkyl, alkoxyl, furyl, substituted furyl, thienyl and substituted thienyl. Examples of xcex2-diketones of use in the present invention are acetylacetone, trifluoroacetylacetone, hexafluoroacetylacetone, thienoyltrifluoroacetylacetone and FOD (a compound in which R1 is t-butyl, R2 is hydrogen, Rxe2x80x22 is hydrogen and R3 is n-C3F7).
The proposed method enables one to extract the metals contaminating the surface with a substantially lower excess of, for instance, a xcex2-diketone than in the case of the known xcex2-diketone method (10-200 mol xcex2-diketone against 400-1000 mol xcex2-diketone per mol metal in the known method). This significantly reduces the costs of the process since xcex2-diketone is quite an expensive reagent.
The carbon dioxide gas can be easily collected and used again in the process. In comparison with the method using, for instance, aqueous solution, the volume in which the radionuclides are collected is 10-100 times smaller and the separation of radionuclides from it is much simpler.
The organic amine is preferably an aromatic or heterocyclic amine and is more preferably a pyridine, a quinoline or an aniline compound.
Preferred amines of use in the present invention include pyridine, an alkylpyridine, quinoline, an alkylguinoline, dipyridine and dimethyl aniline.
Preferably the supercritical fluid additionally contains water.
Following contact between the supercritical fluid and the solid surface, the resultant extract is preferably collected into a suitable solvent. Preferred solvents include water, an aqueous or aqueous organic solvent and an organic solvent.